Vincent Jomelli

GEOMORPHIC VARIATIONS OF DEBRIS FLOWS AND RECENT CLIMATIC CHANGE IN THE FRENCH ALPS

Much work has been done to show that there is a relationship between the triggering of debris flows and the recorded increase in temperatures or in the number of intense rainy events over the last few decades. The question addressed in this paper is that of the impact of these climate changes on the dynamics of debris flows since the 1950s. 319 debris flows in the Dévoluy and Ecrins massifs located in the French Alps, the triggering of which is independent of the current glacial retreat, have been analysed. In the Dévoluy a reduction in the number of debris flows was observed in the periods 1950–1975 and 1975–2000. In the Massif des Ecrins, we have observed a shift of the triggering debris flow zone toward higher elevations and a lack of significant variation in the number of debris flows. But in the Massif des Ecrins this global result masks two different trends depending on elevation. At low altitude (<2200 m) the number of debris flows and the frequency of debris flows less than 400 m in length have decreased significantly since the 1980s whereas no significant variation was observed at high altitude (>2200 m). At the same time, we have observed a significant increase in the annual and seasonal temperatures for these 20 last years combined with a significant reduction in the number of freezing days. A significant increase in summer rains higher than 30 mm/d has also been observed. In Dévoluy and at low altitude in the Massif des Ecrins,these variations can be explained by the decrease in the number of freezing days related to the increase in the temperatures, which implies a slower reconstitution of the volume of debris stored between two events. But at high altitude it is currently difficult to establish the link between the climatic change and the dynamics of the debris flows because very little is known about the two variables controlling the triggering of the debris flows, i.e., on the one hand intense precipitations and on the other hand the volume of rock debris.

Comparing the characteristics of rockfall talus and snow avalanche landforms in an Alpine environment using a new methodological approach: Massif des Ecrins, French Alps

Abstract The geometry and sedimentology of 49 talus slope deposits have been analysed in the French Alps to show the effects of snow avalanche activity on their characteristics. Twenty slopes are heavily avalanche-modified, 15 are snow-avalanche boulder tongues and 14 are rockfall-talus slopes. Slope angle, slope segmentation, longitudinal and sorting indices provide criteria to differentiate these deposits. Although the transition from rockfall- to avalanche-dominated slopes is gradational, a “transitional talus” is identified with distinctive characteristics.

Wet snow avalanche deposits in the French Alps: Structure and sedimentology

We analyse the morphology and sedimentology of 25 dirty snow avalanche deposits in the French Alps. The deposits typically have either a snow‐ball structure or a massive structure with sliding planes. The snow balls show a longitudinal and a vertical sorting that reflects a sieve effect, similar to that observed in other rapid inertial granular flows. The massive type results from snow compaction when the avalanche is channelled by a gully or when it reaches the distal part of the scree. Velocity decrease and compaction limit the deformation to a zone at the base of the snow mass and cause the formation of distinctive sliding planes. These appear as smooth recrystallised surfaces due to local melt from frictional heating. The flow can be assimilated to a frictional granular flow. No systematic variation of size and shape of the rock debris has been observed along the profiles in both types of deposit. The distribution of rock debris and its fabric suggest that the clasts are transported passively and do not undergo any sorting during displacement. Snow melt after avalanching causes a redistribution of rock debris particularly when the snow thickness is important. This redistribution does not generate new sedimentological characteristics such as enhanced sorting or fabric.

Irregular tropical glacier retreat over the Holocene epoch driven by progressive warming

The causes and timing of tropical glacier fluctuations during the Holocene epoch (10,000 years ago to present) are poorly understood. Yet constraining their sensitivity to changes in climate is important, as these glaciers are both sensitive indicators of climate change and serve as water reservoirs for highland regions. Studies have so far documented extra-tropical glacier fluctuations, but in the tropics, glacier–climate relationships are insufficiently understood. Here we present a 10Be chronology for the past 11,000 years (11 kyr), using 57 moraines from the Bolivian Telata glacier (in the Cordillera Real mountain range). This chronology indicates that Telata glacier retreated irregularly. A rapid and strong melting from the maximum extent occurred from 10.8 ± 0.9 to 8.5 ± 0.4 kyr ago, followed by a slower retreat until the Little Ice Age, about 200 years ago. A dramatic increase in the rate of retreat occurred over the twentieth century. A glacier–climate model indicates that, relative to modern climate, annual mean temperature for the Telata glacier region was −3.3 ± 0.8 °C cooler at 11 kyr ago and remained −2.1 ± 0.8 °C cooler until the end of the Little Ice Age. We suggest that long-term warming of the eastern tropical Pacific and increased atmospheric temperature in response to enhanced austral summer insolation were the main drivers for the long-term Holocene retreat of glaciers in the southern tropics.

Glacier recession on Cerro Charquini (16°s), Bolivia, since the maximum of the Little Ice Age (17th century)

Cerro Charquini, Bolivia (Cordillera Real, 5392 m a.s.l.) was selected as a site to reconstruct glacier recession since the maximum of the Little Ice Age (LIA) in the central Andes. Five glaciers, located on differently exposed slopes, present comprehensive and well-preserved morainic systems attributed to former centuries. The moraines were dated by lichenometry and show a consistent organization on the different slopes. The past geometry of the glaciers was reconstructed using ground topography and aerophotogrammetry. Lichenometric dating shows that the LIA maximum occurred in the second half of the 17th century, after which the glaciers have receded nearly continuously. Over the last decades of the 20th century (1983-97), recession rates increased by a factor of four. On the northern and western slopes, glaciers receded more than on the southern and eastern slopes (by 78% % and 65% % of their LIA maximum area, respectively). The mean equilibrium-line altitude (ELA) rose by about 160 m between the LIA maximum and 1997. Recession rates were analysed in terms of climatic signal, suggesting that glacier recession since the LIA maximum was mainly due to a change in precipitation and that the 19th century may have been drier. For the 20th century, a temperature rise of about 0.68C appears to be the main cause of glacier recession. Recent climatic conditions from 1983 to 1997 correspond to a mass deficit of about 1.36 m w.e. a -1 . If such conditions persist, the small glaciers below 5300 m a.s.l. in the Cordillera Real should disappear completely in the near future.

Unstable ice stream in Greenland during the Younger Dryas cold event

Past, present, and future ice sheet stability is closely linked to the dynamic behavior of major draining ice streams and surrounding ice shelves. While short observational records document the recent variability and acceleration of ice streams, the long-term dynamics of ice streams remain poorly documented. Here, we date the Pjetursson’s Moraine on Disko Island, Greenland, to 12.2 ± 0.6 ka and demonstrate that the Jakobshavn Isbrae (JI) ice stream collapsed during the middle of the Younger Dryas (YD) cold interval. We suggest that this collapse was due to the incursion of warm subsurface water under the ice shelf fronting the JI ice stream, as well as increased surface-air temperature and sea-surface temperature seasonality starting at the beginning of the YD cold interval. The triggered acceleration of the land-based JI and the delivery of icebergs into Disko Bugt potentially contributed to Heinrich Event 0 at the end of the YD. The collapse of the JI ice stream 12.2 ± 0.6 ka ago demonstrates that calving marine-based ice margins can respond rapidly to environmental changes. It provides a new benchmark for marine-terminating ice stream models.

Modeling Uncertainties in Lichenometry Studies

ABSTRACT To date glacial and periglacial landforms, lichenometry is a valuable method but, to improve efficiency, the estimated surface dates derived from traditional methods need to be more accurate. In other words, the statistical uncertainty associated with inferred dates has to be reduced. How to perform such a reduction is the main question that we will address in this paper. An interdisciplinary approach (lichenometry and statistics) allows reduction in the main sources of uncertainty: lichen diameters and their associated ages. Around 2600 lichen measurements collected on moraines from the Charquini glacier in Bolivia (Cordillera Real) are used to illustrate the advantages of our approach over past studies. As for any statistical estimation procedure, the error analysis in lichenometry is directly linked to the type of observations and the statistical model used to represent accurately these data. The attribute of lichenometry studies is that the measurements are not averages but maxima; only the largest lichen diameters provide information about the surface ages. To take this characteristic into account, we propose a novel statistical way to model maximum lichen diameters. Our model, based on the extreme value theory, allows us to compute small confidence intervals for the inferred surface ages. In addition, it offers three other advantages: (1) a global statistical model, as all our data (dated surfaces and all lichen maximum diameters) are represented with a unique function; (2) a mathematical framework within which the maximum lichen distribution is derived from a statistical theory; and (3) flexibility, as different types of growing curves can be investigated.

Evidence of moist niches in the Bolivian Andes during the mid-Holocene arid period

To examine the climate of the mid-Holocene and early human settings in the Andes when the Altiplano was recording the most arid phase of the Holocene, we analyzed plant-related proxies (pollen, phytoliths, diatoms, stable isotopes) from a sediment core sampled at high elevation in the Eastern Cordillera of Bolivia. Our study was carried out in the wetland of Tiquimani (16°12′06.8″S; 68°3′51.5″W; 3760 m), on a well-known pathway between Amazonia and Altiplano. The 7000-year old record shows a two-step mid-Holocene with a dry climate between 6800 and 5800, followed by a wetter period that lasted until 3200 cal. yr BP. In the Central Andes of Bolivia, a widespread aridity was observed on the Altiplano during the mid-Holocene. However, here, we show that moisture was maintained locally by convective activity from the Amazon lowlands. During the arid interval between 5000 and 4000 yr BP, these niches of moisture produced specific grasslands that may have enabled the survival of an archaic culture of hunter–gatherers on the Puna. This development occurred 2000 years before expansion of quinoa cultivation on the Puna.

A New Tree-Ring-Based, Semi-Quantitative Approach for the Determination of Snow Avalanche Events: use of Classification Trees for Validation

Abstract On forested paths, dendrogeomorphology has been demonstrated to represent a powerful tool to reconstruct past activity of avalanches, an indispensable step in avalanche hazard assessment. Several quantitative and qualitative approaches have been shown to yield reasonable event chronologies but the question of the completeness of tree-ring records remains debatable. Here, we present an alternative semi-quantitative approach for the determination of past snow avalanche events. The approach relies on the assessment of the number and position of disturbed trees within avalanche paths as well as on the intensity of reactions in trees. In order to demonstrate that no bias was induced by the dendrogeomorphic expert, we carry out a statistical evaluation (Classification and Regression Trees, or CART) of the approach. Results point to the consistency and replicability of the procedure and to the fact that the approach is not restricted to the identification of high-magnitude avalanches. Evaluation of the semi-quantitative approach is illustrated on a well-documented path in Chamonix, French Alps. For the period 1905–2010, comparison between the avalanche years recorded in a substantial database (Enquête Permanente sur les Avalanches, or EPA) and those defined with dendrogeomorphic techniques shows that the avalanche record reconstructed from tree-ring series contains 38% of the observed events.

Can we infer avalanche–climate relations using tree-ring data? Case studies in the French Alps

Abstract Dendrogeomorphology is a powerful tool to determine past avalanche activity, but whether or not the obtained annually resolved chronologies are sufficiently detailed to infer avalanche–climate relationships (in terms of temporal resolution) remains an open question. In this work, avalanche activity is reconstructed in five paths of the French Alps and crossed with a set of snow and weather variables covering the period 1959–2009 on a monthly and annual (winter) basis. The variables which best explain avalanche activity are highlighted with an original variable selection procedure implemented within a logistic regression framework. The same approach is used for historical chronologies available for the same paths, as well as for the composite tree-ring/historical chronologies. Results suggest that dendrogeomorphic time series allow capturing the relations between snow or climate and avalanche occurrences to a certain extent. Weak links exist with annually resolved snow and weather variables and the different avalanche chronologies. On the contrary, clear statistical relations exist between these and monthly resolved snow and weather variables. In detail, tree rings seem to preferentially record avalanches triggered during cold winter storms with heavy precipitation. Conversely, historical avalanche data seem to contain a majority of events that were released later in the season and during episodes of strong positive temperature anomalies.